For many RF safety and regulatory applications there is a need to measure the RF field strength independently from the electro-magnetic field orientation in each X, Y and Z dimension. This requires an isotropic (omnidirectional) antenna. However, a single such antenna (to measure 3-dimensions) physically doesn't exist.
Many single axis antennas are called “isotropic” though they are not. For example, monopole antenna oriented in the direction Z, is “isotropic” if field source is rotated around Z-axis in the plane X-Y and field polarization coincides with antenna axis Z. But for the field polarization in the plane X-Y, signal of this antenna is zero, therefore such antenna is not isotropic.
In an attempt to overcome the single-plane limitation of a single-axis antenna (Ez), a user manually orients it in space to maximize the signal so that effectively E=Ez. However, for each frequency, in general, there will be a different antenna orientation producing the maximum signal. Therefore, for multiple signal sources and/or broad frequency range scanning this method becomes time consuming and actually inaccurate because it requires the operator to continuously manipulate the antenna at each frequency of interest. Also there is a possibility to skip some important signals due to unfavorable antenna orientation. The same techniques are applied to the magnetic field antennas, using instead a small single axis antenna loop antenna, where ‘small’ means size compared to the received signal wavelength.
More sophisticated method of creating the system, producing the response equivalent to the isotropic antenna, includes positioning three separate single axis antennas at mutually orthogonal positions (Ex, Ey, Ez) and then calculating the resulting field as E=SQRT(Ex2+Ey2+Ez2). Such approach requires a complicated antenna system with three identical RF channels of signal processing. An example 40 is shown in FIG. 1 where there are 3 orthogonal (X, Y and Z) antennas 42, 44, 46, and corresponding amplifiers 43, 45, 47, respectively, having a connection to the RF signal processing circuit 48, Corresponding Ex, Ey, Ez components of the field at a given point of space are measured sequentially in time via an RF Switch 50 having an output to EMI receiver 52 which provides an output to a readout circuit 54 synchronized with the RF switch 50 according to multiplexer control 56. The amplitude of the field magnitude E=SQRT(Ex2+Ey2+Ez2) is calculated by readout circuit 54 and displayed on display 58 along with the frequency of the signal selected by the EMI receiver. This antenna meets the “omnidirectionality” requirements for RF signals that change their amplitude slower then our ability to measure and calculate the field module.
If a broad spectrum of the field strength E at any frequency is to be scanned and displayed, the EMI receiver is replaced with spectrum analyzer. The problem that arises here is that antennas multiplexer and signal processing multiplexer need to be synchronized, that is when antenna X is connected to the antenna output the same “X” sampling circuit should be selected to measure and store the value of the Ex component of the field as shown in the example 60 of FIG. 2. The antenna module 62 contains three orthogonal 42, 44, 46 antennas, corresponding amplifiers 63, 65, 67 and RF switch 64 controlled by the multiplexer control circuit 76, located in the spectrum analyzer 70. The RF output from the RF switch 64 in the antenna module 62 comes through cable 66 while the multiplexer control circuit 76 control signal is sent to the antenna module 62 (and to the RF switch 64) though a separate cable or other separate connection. When spectrum analyzer 70, including a receiver 72 and readout circuit 74 (controlled by the multiplexer controller 76) sweeps the frequency range, at each frequency the components Ex, Ey and Ez are measured and the calculated field strength magnitude E is displayed. This approach has some disadvantages in that although system 60 imitates the performance of an isotropic antenna, it requires a special spectrum analyzer, with a special, separate second cable.
Considering that vast majority of the existing spectrum analyzers can't provide the special signal processing required by this antenna example 60 of FIG. 2, there continues to be a need for the omnidirectional antenna that has an RF output that can work with. ANY spectrum analyzer without the requirement of a special multiplexer controlled display and computation, and a second special cable to operate.